Semiconductive devices are widely used as electronic switches. Besides the usual bipolar transistors, field-effect transistors (FETs) can be used for this purpose, especially those of the MOS (metal-oxide-semiconductor) type known as MOSFETs. With transistors of this nature it is possible to combine a multiplicity of switching circuits into an integrated and miniaturized module also including associated impedance elements such as resistors and capacitors.
In a system requiring complete cutoff of a signal path, however, the inherent capacitance of the MOSFET constitutes a residual admittance whose presence prevents complete insulation of the load from the signal input. In a time-division-multiplex (TDM) telecommunication system, for example, such incomplete insulation gives rise to cross-talk between the individual communication links.
Another problem heretofore encountered in such switching circuits is the need for an ancillary or pilot transistor to which a control voltage independent of the input signal is applied for turning the main or switching transistor on and off, this pilot transistor normally drawing a certain amount of current even in the nonconducting state of the switching transistor. Since in a TDM system each switching transistor generally is to conduct only during a small fraction of a cycle, the dissipation of energy by the associated pilot transistors is particularly wasteful.